1. Field of the invention
The invention relates to a method for adjusting an optical system of a microlithographic projection exposure apparatus.
2. Prior art
Microlithography is used for producing microstructured components such as e.g. integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus, which has an illumination device and a projection lens. Here, the image of a mask(=reticle) illuminated via the illumination apparatus is projected via the projection lens onto a substrate (e.g. a silicon wafer), which is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure onto the light-sensitive coating of the substrate.
During the operation of a microlithographic projection exposure apparatus, there is the need to set defined illumination settings, i.e. intensity distributions in a pupil plane of the illumination device, in a targeted manner. To this end, in addition to the use of diffractive optical elements (so-called DOEs), the use of mirror arrangements has also been disclosed, for example in WO 2005/026843 A2. Such mirror arrangements comprise a multiplicity of micro-mirrors which can be set independently of one another.
Furthermore, a number of different approaches are known for setting, in the illumination device, specific polarization distributions in the pupil plane and/or in the reticle in a targeted manner for the purpose of optimizing the imaging contrast. In particular, it is known to set a tangential polarization distribution in both the illumination device and the projection lens for the purpose of contrast-rich imaging. “Tangential polarization” (or “TE polarization”) is understood to mean a polarization distribution in which the oscillation planes of the electric field strength vectors of the individual linearly polarized light rays are oriented approximately perpendicularly to the radius directed at the optical system axis. By contrast, a “radial polarization” (or “TM polarization”) is understood to mean a polarization distribution in which the oscillation planes of the electric field strength vectors of the individual linearly polarized light rays are oriented approximately radially to the optical system axis.
In respect of the prior art, reference is made, for example, to WO 2005/069081 A2, WO 2005/031467 A2, U.S. Pat. No. 6,191,880 B1, US 2007/0146676 A1, WO 2009/034109 A2, WO 2008/019936 A2, WO 2009/100862 A1, DE 10 2008 009 601 A1, DE 10 2004 011 733 A1 and US 2011/0228247 A1.
Here, a possible approach for flexible setting of the polarization distribution comprises the use of a polarization-influencing optical arrangement made of a plurality of polarization-influencing components, which are arranged such that they are displaceable transversely with respect to the light-propagation direction, in combination with a mirror arrangement which comprises a multiplicity of mirror elements which can be adjusted independently of one another. Here, depending on the degree of cover of the mirror arrangement by the polarization-influencing components, in conjunction with a likewise variable setting of the mirror elements of the mirror arrangement, it is possible, in a flexible manner, to realize different polarization distributions in the pupil plane of the illumination device.
However, in the case of this approach, the problem that can occur in practice is that if the polarization-influencing components are not positioned precisely, one example being the inadvertent partial or complete cover of one or more mirror elements by one or more polarization-influencing components of the polarization-influencing arrangement, then components of the light reflected in the pupil plane by the mirror arrangement are actuated with an incorrect polarization state, and so the polarization distribution obtained in the pupil plane deviates from the desired polarization distribution. Overall, this can result in an adverse effect on the performance of the projection exposure apparatus as a result of imaging errors, and in a loss of contrast.